Method and apparatus for expanding and conferring a cup shape to the terminal junction segment of bi-axially oriented pipes made of thermoplastic material

ABSTRACT

The invention provides for conferring a cup shape to the terminal junction segment ( 2 ) of pipes that are bi-axially oriented longitudinally and circumferentially and hence very sensitive to diameter and length reduction through heat by means of passage into a furnace ( 4 ) which heats the segment ( 2 ) to a differentiated temperature, increasing towards the end of the segment ( 2 ), whose inner diameter progressively drops down to a controlled value, as temperature increases (whilst length is simultaneously reduced, with a corresponding increase in thickness). Preferably then, in an appropriate station ( 33 ), an additional heating is executed to a plastic deformation temperature suited to obtain a correct preliminary dilation of the terminal segment ( 2 ), introducing a rigid element ( 34 ) which acts as inner contrast, thereby inhibiting any retraction thereof. Here the segment ( 2 ) undergoes a thickening of the dilated wall both during the introduction of the rigid element ( 34 ), and during its extraction therefrom. Such terminal segment ( 2 ), dilated and heated, is then easily and rapidly fitted onto a calibration expander ( 3 ), itself also heated to such a temperature as to limit heat absorption from said segment ( 2 ). Here too the terminal end ( 2 ) thickens yet again and takes its shape and, after the possible additional heating to force the terminal segment ( 2 ) to adhere spontaneously and perfectly to the underlying expander ( 3 ), at least the outer surface of the terminal segment ( 2 ), thus shaped, is cooled.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an enhanced method for expandingand conferring a cup shape to the terminal junction segment ofbi-axially oriented pipes made of thermoplastic material, by means of acalibration expander, able to expand the terminal segment, previouslyheated to the plastic state, shaping therein an annular circumferentialseat for housing a corresponding sealing gasket for the junction.

[0002] As is well known, apparatuses that operate on thermoplasticmaterials such as PVC, polyethylene (PE) and/or polyolefins, which arenot bi-axially oriented, comprise furnaces for the prior heating of theterminal segment. This pipe portion is brought to a temperature of about120°C. or more, to obtain its adequate softening and enable then easilyto fit it on the calibration expander, which dilates it and shapes itinto the so-called “cup” conformation, to obtain an effective junctionbetween the pipe thus treated and the end of another pipe, at thenominal diameter, once between the two ends is interposed the annularsealing gasket, stably positioned in the annular seat obtained in saidterminal segment. Within the scope of this technology the calibrationexpander, in this case said to be the mechanical expansion type, maypresent a crown of radially expandable sections able to be retracted oncommand, with the purpose of shaping the annular seat of the terminalsegment when there are expanded, and to let the expander easily exit theshaped terminal segment, then are retracted.

[0003] In this case it is evident that the annular gasket shallsubsequently need to be inserted in the annular seat of the shaped andcooled terminal segment.

[0004] Alternatively to the above, the calibration expander may presentan annular groove able simultaneously to retain the annular gasketintroduced therein with a thrust flange which, once the gasket isinserted into the expander, causes it to be housed in the annulargroove. Once this is accomplished, the phase of shaping the terminalsegment starts, with the necessary aid of a flange for upsetting andholding the gasket, located downstream thereof. Subsequently, theexpander is extracted, and in this case it can be defined as the freegasket type, after cooling the terminal segment, whilst the gasketremains trapped in the annular seat formed in the terminal segment.

[0005] The aforesaid technologies refer, as stated, to the treatment ofpipes made of thermoplastic material not bi-axially oriented.

[0006] The latter technology entails a considerable reduction in thethickness of pipes made of thermoplastic material, for the sameresistance to internal pressure, since it has been ascertained that, inthese materials, if molecules constituting the material are stretched orelongated in a same direction or in mutually orthogonal directions, thethickness of the material decreases, but not its resistance to theinternal pressure designed for the untreated pipe.

[0007] Thus technologies have been developed for the bi-axialorientation of the thermoplastic material, entailing a circumferentialdilation of the pipe and its elongation or stretching in the axialdirection. For example the international patent applications WO95/25626, WO 95/25627, WO 95/25268, WO 95/30533 are mentioned. In thismanner, for the same quantity or volume of material it is possible toproduce a greater number of linear meters of pipe, which still meets thepressure resistance requirements originally prescribed.

[0008] However, such materials have an intrinsic characteristic whichnegatively reflects on the final product; if the extruded pipe initiallyhas a certain diameter and, after the bi-axial orientation process, aclearly greater diameter which is the nominal one to be obtained, ittends drastically to reduce in diameter, if subjected to temperatures ofa certain level, such as those able to soften the material for theprocesses whereby the terminal junction segment is formed according tothe technologies illustrated above for pipes in non bi-axialthermoplastic material.

[0009] This is because the molecules of the thermoplastic materials thatare subjected to diameter change “remember” the previous physical stateand tend to return to the original state.

[0010] Hence, if a pipe is extruded to a certain diameter which issubsequently increased, in a bi-axial orientation treatment, if it weresubjected to the temperatures (120°C. and higher) of the aforesaidfurnaces its diameter would drastically decrease.

[0011] The aforesaid technologies, perfectly suited to shape theterminal segments of pipes made of thermoplastic material not subjectedto bi-axial orientation processes or anyway particular subsequentdiameter dilations, are therefore not at all applicable to pipes made ofbi-axially oriented thermoplastic material.

[0012] The bi-axially oriented pipe would lose its thinnesscharacteristics, tending to return to the original thickness, andtherefore could not be fitted in the expander set for the useful nominaldiameter, corresponding to the diameter of the bi-axially oriented pipe.

[0013] Moreover, even if it were possible to reduce the heatingtemperatures of the thermoplastic material to limit diameter reduction,the end segment thus obtained still would not be able to fit on theexpander, because it would be too cold and thus would exert anexcessively high progressive friction.

[0014] The applicant has also observed that, since there is a need forthe temperature within the differentiated temperature furnace to belimited, in order to prevent the inlet to the terminal segment to bereduced excessively and to retain insofar as possible unaltered thebi-axial orientation characteristics, there could be a risk of damagingthe terminal segment during a particularly stressful use of the treatedpipe. Moreover, the shaping phase on the expander, though appropriatelyheated, may become excessively onerous, for pipes of considerable sizeand technical characteristics of resistance, both in terms of effortrequired, and of operating times, thus penalizing the hourly productionof the pieces.

SUMMARY OF THE INVENTION

[0015] The aim of the present invention is to eliminate all thedrawbacks mentioned abode, providing a method and an apparatus which,whilst using also previously known elements, solves the problems ofeffectively and simply shaping the terminal junction segment of pipesmade of bi-axially oriented thermoplastic material, and also allows toincrease hourly production of such treated pipes, reducing the durationof the longest working phase, which is the final shaping of the terminalsegment on the calibration expander and guaranteeing the technicalfeatures of the product.

[0016] The invention, as it is characterized in the claims, with aparticular and original heat treatment of the terminal segment of thebi-axially oriented pipe, allows it to receive a particular shape,tapered and converging at the tip. Moreover, the pipe, by reducing itslength increases its thickness just where this is very useful, theterminal segment being subjected to higher radial stresses, due to itsdiameter, greater than the nominal diameter of the pipe.

[0017] Furthermore, heating to an appropriate temperature the expanderas well, allows the terminal segment not to cede heat in any way to theexpander itself.

[0018] Under such conditions, the tapered terminal segment can be fittedon the expander, with acceptable friction and anyway favored by the factthat the expander itself, when it penetrates in the terminal segment,finds increasing diameters.

[0019] In any case, during this phase, the terminal segment thickensfurther, allowing it to reach thicknesses that are able easily towithstand the nominal design pressures in such segments.

[0020] Furthermore, the invention, with a particular and originaladditional heat treatment of the terminal segment of the bi-axiallyoriented pipe, allows its easy and more rapid shaping, while alsoeliminating any risk of subsequent drawbacks in correspondence with theterminal segment and maintaining excellent characteristics ofbi-orientability and perfect resistance to nominal design pressures.

[0021] Additionally, with the subject invention, it is also possible tomodify and/or control the thickness of the terminal segment, allowing toreach even greater thicknesses, optimal easily to withstand the nominaldesign pressures in such segments, once again with no need, as was thecase in prior art processes, preventively to thicken the terminalsegments.

[0022] Moreover, to guarantee under any condition the perfect adherenceof the terminal segment to the underlying expander, in order to obtainexcellent precision and internal conformations of the shaped segment,use is also made of a heat flash obtained from an electrical coilpositioned around the terminal segment, which forces the segment itself,until it is internally contrasted by the metallic expander, strongly toadhere thereto

[0023] An appropriate cooling, which can also be superficial in thiscase, concludes the process.

[0024] Both the subject method and apparatus can use either a mechanicalexpansion expander, or free gasket expanders.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Further features and advantages of the invention shall becomemore readily apparent in the detailed description that follows,illustrated purely by way of non-limiting example in the accompanyingdrawings, in which:

[0026]FIG. 1 shows at the top a terminal junction segment of bi-axiallyoriented pipe and, at the bottom, a progressive heating furnace with theconsequent plastic deformation of the aforesaid terminal segment;

[0027]FIG. 2 shows the apparatus relating to a free gasket expander,with some details;

[0028]FIG. 3 shows the apparatus of FIG. 2 with the gasket inserted onthe expander and with other details;

[0029]FIG. 4 shows the phase of progressively and forcedly introducingthe expander inside the terminal segment of the pipe, with a furtherheating phase of the exterior of the terminal segment itself;

[0030]FIG. 5 shows a final cooling phase of the terminal segment;

[0031]FIG. 6 shows the extraction of a free gasket expander from theterminal segment already treated according to the invention;

[0032]FIG. 7 shows the extraction of a mechanical expansion expanderfrom the terminal segment already treated according to the invention;

[0033]FIG. 8 shows the method and the apparatus according to a preferredembodiment of the invention, with a partially sectioned view and withsome parts removed the better to highlight others;

[0034]FIG. 9 shows a part of the invention of FIG. 8, in one of itscharacteristic phases.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] The subject intention relates to a method for expanding andconferring a cup shape to the terminal junction segment of bi-axiallyoriented pipes made of thermoplastic materials. As previously stated, inknown methods and apparatuses, in order to execute the aforesaid shapingin non bi-axially oriented thermoplastic pipes, the use is known of acalibration expander (3), able to expand the terminal segment (2),previously heated to the plastic state, shaping its circumferentialannular seat (21) for housing a corresponding sealing gasket (22) forthe junction. Such calibration expander can be of the mechanicalexpansion type or of the type defined above as with free gasket. In thecase at hand, either one of such expanders can be used. In the firstcase (see FIG. 7, where an expander (3) with expandable sectors (32) inthe retracted position is shown) it shall still be necessary to insertthe gasket (22) inside the annular seat (21), after the end ofoperations and the exit of the expander from the terminal segment (2),whereas in the second case, as FIG. 6 clearly shows, this shall not benecessary, since the gasket (22) is already in its seat and therein itwill remain, even after the expander is extracted (3). Conversely, inthis latter case technical elements shall be necessary, which shall beillustrated further on and which are not necessary in the first case.

[0036] The example illustrated herein which follows shall refer to theuse of an expander of the second type illustrated above.

[0037] Preventively (see FIG. 2), a thrust flange (9), movable in thetwo opposite directions (d1-d2), thrusts, in the direction (d2), thegasket (22) previously inserted on the expander (3), into the annulargroove (10) of the expander (see FIG. 3).

[0038] The subject method or process comprises the phases listed below.

[0039] A first phase entails heating in a furnace (4) the terminalsegment (2) of the bi-axially oriented pipe (1), to a differentiatedtemperature, increasing towards the end of the terminal segment (2). Inthe experimentations conducted by the applicant, temperatures varywithin a non-limiting range that is sharply lower than the (constant)ones located in the usual furnaces to soften the segments (2) made ofthe usual, non bi-axially oriented, thermoplastic material (120 decreescentigrade and more). Such temperatures must also be controlled so thatthe inner diameter (DN) of the terminal segment (2) progressivelyreduces with the increase of the temperature, which becomes the maximumtemperature (Tmax), at the end of the terminal segment (2). The toplimit of said maximum temperature is defined by the need for the minimuminner diameter (Dmin) of the terminal segment (2) to be greater than theminimum diameter (dmin) of the front end of the expander (3), so theexpander (3) can be introduced in the terminal segment (2).

[0040] A second phase entails heating the calibration expander (3) to atemperature (TM) that exceeds or equals the maximum temperature (Tmax)located at the end of said terminal segment (2), to prevent the expanderfrom removing heat from the terminal segment (2) during the subsequentphase of progressively and forcedly introducing the heated expanderinside the terminal segment (2) of the pipe (1), phase which thusentails no heat transfer from the terminal segment (2) of the pipe tothe expander. Such introduction is enabled and facilitated by theprogressive diameter increase of the terminal segment (2) whilst theexpander (3) progressively penetrates therein.

[0041] A subsequent phase entails cooling at least the outer surface ofthe shaped terminal segment (2), prior to extracting said expander (3).It has been noted that even a single outer surface cooling of the pipewould suffice to avoid subsequent spontaneous deformations after theexpander (3) is extracted.

[0042] In the preferred embodiment of the invention, the upper limit ofsaid maximum temperature is defined by the need for the minimum innerdiameter (Dmin) of the terminal segment (2) to be greater than theminimum diameter (dmn) of the front end of a rigid element (34) whereonmore shall be stated further on, instead of the expander (3). Accordingto this embodiment, after this phase, a preliminary phase is providedwhereby the terminal segment (2) is further heated to a plasticdeformation temperature suited to obtain a correct circumferentialdilation of the terminal segment (2), introducing a rigid element (34)also heated and acting as an inner contrast, thereby inhibiting, atleast during such heating, any possibility of retraction by the terminalsegment (2), contributing to its dimensional stabilization and therebyfavoring its subsequent introduction on the calibration expander (3).

[0043] It is advantageous that the aforesaid preliminary dilation andheating phase occurs at a differentiated temperature, increasing as theend of the terminal segment (2) is approached and with an average valuegreater than that measurable in the furnace (4), in order to facilitateto the highest possible degree the subsequent introduction on thecalibration expander (3), making the parts downstream of the end of theterminal segment (2) progressively more resistant

[0044] In this case, the upper limit of the maximum temperature (Tmax),located at the end of the terminal section (2) inside the furnace (4) issuch that the minimum inner diameter (Dmin) of the terminal section (2)must be greater than the minimum diameter (dmn) of the front end of therigid element (34).

[0045] Simultaneously, the phase is provided of heating the calibrationexpander (3) to a temperature that minimizes heat subtraction from theterminal segment (2) during the progressive forced introduction of theheated expander (3) inside the terminal segment (2) of the pipe (1)preventively treated according to the invention.

[0046] Such introduction, thanks to the aforesaid preliminary phase, isquite facilitated and the expander (3) essentially has only the task ofshaping the annular seat (21), with the material at the most suitabletemperature.

[0047] A subsequent phase provides for cooling at least the outersurface of the shaped terminal segment (2), prior to extracting saidcalibration expander (3).

[0048] It is interesting to note that, during the first phase, areduction in the length (L1) of the terminal segment (2) occursspontaneously, for instance by a measure (ΔL), as shown in FIG. 1, whichcorrespondingly causes an increase in the thickness of the thermoplasticmaterial in said segment (2). Moreover, during the forced introductionof the expander (3) into the terminal segment (2) a phase whereby thewall of the terminal segment (2) thickens automatically sets in, whichleads to a final thickness (S2) that is considerably greater than theinitial thickness (S1) of the pipe, also with the decisive contributionof what occurs during said preliminary phase. In the preliminary phaseconstituting the subject of the invention, a considerable thickening ofthe wall of the terminal segment (2) automatically sets in because therigid element (34), of greater diameter than the inner diameter (DN) ofthe terminal segment (2) is movable in the two directions (d4, d5),since it has first to be forcedly and progressively introduced, in thedirection (d4) inside the terminal segment (2), held locked in place,and then retracted, in the opposite direction (d5), from the segment,prevented from lengthening, in the same direction, being blocked by astop (82), fixed with respect to the rigid element (34). Hence both theintroduction of the terminal segment (2), and the extraction of therigid element (34) cause the material to thicken.

[0049] All this thus allows completely to eliminate operations for thepreventive thickening of the terminal segment (2), since such thickeningtakes place spontaneously and sufficiently, thanks to the two aforesaidphases.

[0050] From the point of view of the method, it is preferable also toprovide, after the phase whereby the expander (3) is forcedly introducedinto the terminal segment (2) and prior to cooling, an intermediatephase able to develop an additional flash of direct heat on the terminalsegment (2), as FIG. 4 clearly shows, thereby inducing such segment tocontract spontaneously and to adhere closely on the underlying expander(3), to match its shape perfectly. In particular, prior to this phase, aflange (8) for upsetting and holding in place the gasket (22) is movedrear ward, as shown in FIG. 4, to allow also the retraction of the outersegment (23) which had previously climbed onto the foot of the upsettingflange (8), see dashed line in FIG. 4.

[0051] The apparatus of the invention comprises, in addition to saidfurnace (4), with the aforementioned characteristics, the calibrationexpander (3), which in particular comprises a long tapered segment (31)of the front part, able to facilitate the forced introduction of theexpander (3) inside the terminal segment (2).

[0052] It also comprises:

[0053] heating means (5), which are constituted by at least anelectrical resistor (51) introduced inside the expander, as FIG. 3clearly shows, able to heat the calibration expander (3) to atemperature useful to minimize heat subtraction;

[0054] means (6) for cooling at least the outer surface of the shapedterminal segment (2), prior to the extraction of said expander (3).

[0055] The latter are in practice constituted at least by devices (61)for blowing cooling air. In particular, the blowing devices (61) arepositioned around the expander (3) and supported by said upsettingflange (8), to distribute the cooling air as uniformly as possible alongdirections (d3) tangential to the outer surface of the shaped terminalsegment (2), as shown in FIG. 5.

[0056] It is in any case possible to provide additional cooling meanssuch as a loop with a cooling fluid (62), introduced inside the expander(3).

[0057] The subject invention aims, as stated in the description of themethod, to improve the intrinsic characteristics of the terminal segmentand to quicken its shaping into a cup. The apparatus comprises a station(33) for the further preliminary heating and circumferential dilation ofthe terminal segment (2), before it is fitted onto the expander (3). Thestation is provided with the rigid element (34) able to be forcedly andprogressively introduced inside the terminal segment (2), locked bybilateral clamps (11), after the optimal heating of the rigid element(34) by its own heating means (55), until allowing a temperature ofcorrect plastic deformation of the terminal segment (2). The rigidelement (34) has a minimum front diameter (dmn) which progressivelygrows and then remains constant on a value (DE) corresponding to thedefinitive one of the shaped terminal segment (2). The station (33)advantageously comprises a second furnace (44) with differentiatedtemperature increasing towards the interior of the furnace itself and ofan average value greater than that measurable inside the previousfurnace (4). Movements are regulated by the fact that the station (33)is sliding supported and the rigid element (34) is movable in the twodirections (d4, d5), with respect to the station (33) itself. Thethickening of the wall of the terminal section takes place, in thiscase, both due to friction during the introduction of the terminalsegment (2) on the rigid element (34), and during the subsequentextraction of the rigid element itself. The station (33) comprises aflange (81), identifying inside the second furnace (44) a stop (82) ofthe forward edge of the terminal segment (2), where it exerts stressduring the extraction, in the direction (d5) of the rigid element (34)of the terminal segment itself. This causes the further thickening ofthe material. It has been observed that through this additionalthickening constant ratios are obtained between the diameter and thecorresponding thicknesses of the pipe in the significant areas, i.e.along the pipe itself, in correspondence with the cylindrical part ofthe terminal segment and in correspondence with the annular seat for thegasket.

[0058] The differentiated temperature is obtained by means of a flow ofcooled fluid which moves from the exterior to the interior of thefurnace. In particular, the rigid element (34) comprises internalducting (35), where the cooled fluid flows, ending in correspondencewith a diffuser (36), located at the free front end of the rigid element(34), which routes the flow of cooled fluid towards the opposite end,until it discharges outwards, through holes (37) obtained along therigid element (34) itself. The diffuser (36), advantageously, isprovided with inclined holes (38), so as to route the flow of cooledfluid along the inner walls of the rigid element (34). To the samepurpose also contributes the fact that the internal wall thickness ofthe rigid element (34) increases towards the interior of said secondfurnace (44).

[0059] The subject apparatus comprises a probe (45), able to regulatethe heat produced by a band resistor (56), positioned circumferentallyaround said second furnace (66). It is also advantageous to provideadditional heating means (7), able to develop a further flash of directheat on the terminal segment (2), thereby inducing such segment tocontract and closely adhere on the underlying expander (3), in order tomatch its shape perfectly, means comprising for instance an electricalcoil (71) wound around the expander (3).

[0060] The invention can be subject to numerous modifications andvariations, without thereby departing from the scope of the inventiveconcept and of the claims that follow.

What is claimed:
 1. Method for expanding and conferring a cup shape to the terminal junction segment of bi-axially oriented pipes made of thermoplastic material, by means of a calibration expander (3), able to expand the terminal segment (2), preventively heated to the plastic state, shaping therein a circumferential annular seat (21) for housing a corresponding sealing gasket (22) for the junction, comprising the following phases: heating in a surface (4) the terminal segment (2) of the bi-axially oriented pipe (2), to a differentiated temperature which increases towards the end of the terminal segment (2) and controlled so that the inner diameter (DN) of the terminal segment itself (2) is progressively reduced as temperature increases, which temperature becomes the maximum temperature (Tmax) at the end of the terminal segment (2); heating the calibration expander (3); progressively and forcedly introducing the heated expander (3) inside the preventively heated terminal segment (2); cooling at least the outer surface of the shaped terminal segment (2), prior to the extraction of said expander (3).
 2. Method according to claim 1 , wherein the terminal segment (2) of the pipe (1) inside the furnace (4), during its differentiated heating, undergoes a phase whereby its length (L1) is reduced by a measure (ΔL), correspondingly increasing the thickness of thermoplastic material in said segment (2).
 3. Method according to claim 1 or 2 , wherein, during the forced introduction of the expander (3) in the terminal segment (2) a phase automatically sets in whereby there is a thickening of the terminal segment (2), which reaches a final thickness (S2) greater than the initial thickness (S1) of the pipe.
 4. Method according to claim 1 , wherein the upper limit of the maximum temperature (Tmax), located at the end of the terminal segment (2) inside the furnace (4) is such that the minimum inner diameter (Dmin) of the terminal segment (2) must be greater than the minimum diameter (dmn) of the front end of the expander (3).
 5. Method according to claim 1 , wherein the heating of the expander (3) takes place at a temperature (TM) that is greater than or equal to the maximum temperature (Tmax) located at the end of the terminal segment (2).
 6. Method according to claim 1 , wherein, prior to the phase of introducing the heated expander (3) inside the terminal segment (2), a preliminary phase is provided whereby the terminal segment is further heated to a plastic deformation temperature suited to obtain a correct circumferential dilation of the terminal segment (2) by means of an inner rigid element (34), thereby also inhibiting, at least during such heating, any possibility of retraction by the terminal segment (2), contributing to its dimensional stabilization and in such a manner as to favor its subsequent introduction on the calibration expander (3).
 7. Method according to claim 6 , wherein the aforesaid preliminary heating and dilating phase also occurs at a differentiated temperature, increasing towards the end of the terminal segment (2) and with an average value greater than that measurable in the furnace (4), in order to facilitate to the highest possible degree the subsequent Introduction on the expander (3), making progressively more resistant the walls downstream of the end of the terminal segment (2).
 8. Method according to claim 6 or 7 , wherein also the rigid element (34) is heated.
 9. Method according to claim 6 or 7 or 8, wherein the thickness of the terminal segment (2) of the pipe (1) progressively increases during its differentiated heating inside the furnace (4), undergoing a phase whereby its length (L1) is reduced by a measure (ΔL), as well as during the forced introduction of the expander (3) in the terminal segment (2), and, lastly, during said preliminary further heating phase, wherein a considerably thickening of the wall of the terminal segment (2) sets in, since the rigid element (34), with greater diameter than the inner diameter (DN) of the terminal end (2) is movable in the two directions (d4, d5) since it has to be forcedly and progressively introduced, in the direction (d4) inside the terminal segment (2), locked in place, and then retracted, in the opposite direction (d5), therefrom, prevented from lengthening, in the same direction, being blocked by a stop (82), which is fixed with respect to the rigid element (34).
 10. Method according to claim 6 , wherein the upper limit of the maximum temperature (Tmax), located at the end of the terminal segment (2) inside the furnace (4) is such that the minimum inner diameter (Dmin) of the terminal segment (2) must be greater than the minimum diameter (dmn) of the front end of the rigid element (34).
 11. Method according to any one of the previous claims, wherein, after the forced introduction of the expander (3) in the terminal segment (2) and prior to cooling, an intermediate phase is provided able to develop additional direct heat on the terminal segment (2), thereby inducing such segment to contract and adhere closely on the underlying expander (3) in order to match its shape perfectly.
 12. Apparatus for expanding and conferring a cup shape to the terminal junction segment of bi-axially oriented pipes made of thermoplastic material, provided with a calibration expander (3), able to expand the terminal segment (2), preventively heated to the plastic state, shaping it with a circumferential annular seat (21) for housing a corresponding sealing gasket (22) for the junction, comprising: a furnace (4) able to heat the terminal segment (2) of the bi-axially oriented pipe (1), to a differentiated temperature that increases approaching the end of the terminal segment (2), whose inner diameter progressively drops down to a controlled value, as the temperature increases: heating means (5) able to heat the calibration expander (3); means (6) for cooling at least the outer surface of the shaped terminal segment (2), prior to the extraction of said expander (3).
 13. Apparatus according to claim 12 , wherein the calibration expander (3) comprises a long tapered segment (31) of the front part, able to facilitate the forced introduction of the expander (3) inside the terminal segment (2), the front end of the expander (3) having a lesser minimum diameter (dmin) than the minimum inner diameter (Dmin) of the terminal segment (2).
 14. Apparatus according to claim 12 , wherein the heating means (5) comprise at least an electrical resistor (51) introduced inside the expander (3).
 15. Apparatus according to claim 13 , wherein the cooling means (6) comprise at least devices (61) for blowing cooling air.
 16. Apparatus according to claim 15 , wherein the cooling means (6) further comprise a loop with cooling fluid (62), introduced inside the expander (3).
 17. Apparatus according to claim 15 , provided with an upsetting flange (8), positioned downstream of the end of the terminal segment (2) of the pipe (1) fitted onto the expander (3), wherein said blowing devices (61) are positioned around the expander (3) and supported by said upsetting flange (8).
 18. Apparatus according to claim 13 , comprising a station (33) for the further preliminary heating and circumferential dilation of the terminal segment (2), before it is fitted on the calibration expander (3), said station (33), being provided at least with a rigid element (34) able to be forcedly and progressively introduced inside the terminal segment (2), locked by bilateral clamps (11), after the optimal heating at least of the rigid element (34) by heating means (55), until allowing a temperature of correct deformation of the terminal segment (2), said rigid element (34) having a front minimum diameter (dmn) that increases progressively and then remains constant, on a value (DE) corresponding to the definitive one of the shaped terminal segment (2).
 19. Apparatus according to claim 18 , wherein the station (33) comprises a second furnace (44) with differentiated temperature increasing towards the interior of the furnace itself and with average value greater than that measurable inside the previous furnace (4).
 20. Apparatus according to claim 18 , wherein the station (33) is supported slidingly and the rigid element (34) is movable in the two directions (d4, d5), with respect to the station (33) itself.
 21. Apparatus according to claim 20 , wherein the station (33) comprises a flange (81), identifying within the second furnace (44) a stop (82) of the front edge of the terminal segment (2), where it exerts stress during extraction, in the direction (d5), of the rigid element (34) of the terminal segment itself.
 22. Apparatus according to claim 19 , wherein said second furnace (44) comprises, within it, a flow of cooled fluid which moves from the exterior to the interior of the furnace, to create its said differentiated temperature.
 23. Apparatus according to any one of the claims from 18 to 22, comprising a probe (45) able to regulate the heat produced by a band resistor (56), arranged circumferentially around said second furnace (44).
 24. Apparatus according to claim 19 , wherein the rigid element (34) has an internal wall thickness which increases towards the interior of said second furnace (44).
 25. Apparatus according to claim 22 , wherein the rigid element (34) comprises a ducting (35) where the cooled fluid flows, ending in correspondence with a diffuser (36), located at the free front end of the rigid element (34), which routes the flow of cooled fluid towards the opposite end, until discharging outwards, through holes (37) obtained along the rigid element (34) itself.
 26. Apparatus according to claim 25 , wherein said diffuser (36) is provided with holes (38) inclined in such a way as to route the flow of cooled fluid along the inner walls of the rigid element (34).
 27. Apparatus according to any of the claims from 12 to 26, comprising further heating means (7), located in correspondence with the expander (3), able to develop additional direct heat on the terminal segment (2), thereby inducing such segment to contract and closely adhere on the underlying expander (3), to match its shape perfectly.
 28. Apparatus according to claim 27 , wherein said further heating means (7) comprise an electrical coil (71) wound around the expander (3). 